1. Field of the Invention
The present invention relates to directional couplers, and more particularly to couplers for microwave applications.
2. Description of the Related Art
Arrays of parallel, coupled lines have two general areas of application in microwave circuits: (a) directional couplers; and (b) filters, delay lines, and matching networks. Directional couplers couple a prescribed amount of power input on a first transmission line to an second (or coupled) transmission line. For example, a 3dB coupler couples one-half of the power transmitted by a first transmission line to a second transmission line--the ratio of the power input to the first transmission line to the power coupled to the second transmission line is usually expressed in dB with increased coupling being represented by a smaller coupling value in dB. Power input on the second transmission line is also coupled to the first transmission line. Directional couplers are useful measurement tools which provide a simple, convenient, and accurate means for sampling microwave energy.
FIG. 1 illustrates the basic construction of a conventional coupled-line directional coupler 10 useful in, for example, microwave applications. Directional coupler 10 includes first and second parallel striplines 12, 14 coupled over multiples of approximately one-quarter wavelength (.lambda./4). Ports A and B are provided at first and second ends 16, 18 of first stripline 12 and ports C and D are provided at first and second ends 20, 22 of second stripline -4. The designation stripline, as used herein, refers to any conductor which has infinite ground planes on both sides of the conductor, including, for example, striplines, suspended striplines, and triplate striplines. The conductor itself may have different shapes, e.g., round or rectangular.
Ports A and B which are located at the first and second ends 16 and 18, respectively, of the first stripline 12 and ports C and D which are located at the first and second ends 20 and 22, respectively of the second stripline 14 will alternatively be referred to using the alphabetic port identifier or the following designations. An "input port" is any port to which a signal is applied. A stripline which has an input port is the "through stripline" and the other stripline is the "coupled stripline." With reference to an input port, a "through port" is at the opposite end of the through stripline from the input port, an "adjacent port" is at the same end of the coupled stripline as the input port, and an "opposite port" is at the opposite end of the coupled stripline from the input port. If a coupler has multiple inputs, and thus more than one input port, more than one designation may apply to a single port. In the coupler shown in FIG. 1, if port A is the input port, port B is the through port, port C is the adjacent port, and port D is the opposite port.
The first and second parallel striplines 12, 14, the through and coupled striplines, respectively, have a specified, small spacing in a coupling region 26. Ports A-C are usually configured for connection to coaxial transmission lines and the outer conductor or ground for each coaxial line is connected to grounded body 28 of coupler 10. Port D terminates the second stripline 14 by interconnecting stripline 14 to the body 28 of coupler 10, which is at ground potential, through resistor 30.
Conventional TEM or quasi-TEM mode directional couplers provide contra-directional coupling. In contra-directional coupling, energy applied to a first stripline at an input port is directionally coupled from the first stripline to the coupled stripline and appears at the adjacent port on the coupled stripline, with a greater amount of power appearing at the adjacent port than the opposite port. For example, energy applied at port A of the first stripline 12 appears at port B of first stripline 12; however, some fraction of the energy will appear at port C of the second stripline 14. The amount of energy appearing at port C of second line 14 depends upon the amount of coupling provided in the design of the coupler. Several factors, including the spacing between the striplines 12, 14, determine the amount of energy that may be transferred from the one line to the other.
The amount of coupling desired for forward power (power flowing in the port A-to-port-B direction) varies with the application. For example, a coupler used to split a signal would use tight coupling, i.e., a large amount of power would be coupled to the coupled stripline. Coupling values of 30dB to 3dB are typically encountered in practice, and coupling of 8dB or better (i.e., coupling values of 8dB to 0dB) is generally referred to as tight coupling.
The directivity of a coupler is calculated as the ratio of the power coupled to the adjacent port to the power coupled to the opposite port, expressed in dB. For example, directivity is an indication of the amount of power appearing at port D, as a fraction of the power appearing at port C, when power is applied at port A, and is a measure of the isolation between port A and port D. It is generally desirable to avoid the loss of power, and thus the ideal directional coupler will have an infinite value of directivity. Values of directivity usually range from 5dB to 30dB.
Directional couplers are useful devices for measuring reflected energy. This is accomplished by applying energy to port B and connecting a device under test at port A. Energy reflected by the device under test will flow in the port A-to-port-B direction and a known fraction thereof will appear at port C.
A conventional 3dB, TEM, air dielectric, stripline coupler comprising a first and a second stripline 36 and 38, respectively, is shown in the schematic diagram of FIG. 2A and in the cross sectional view of FIG. 2B. The first and second striplines 36, 38 are provided between first and second ground planes 40, 42 and are surrounded by an air dielectric. The stripline 36 comprises a pair of end sections 36a and a center section 36b. The stripline 38 comprises a pair of end sections 38a and a center section 38b. The center sections 36b and 38b are in parallel and spaced a distance S.sub.1 apart. The spacing d between the ground planes 40, 42 is approximately 0.045". In coupling region 39 each stripline 36, 38 has a thickness t of 0.006" and a width w of 0.020", and the spacing S.sub.1 between the striplines 36, 38 is 0.0025". At the frequencies at which a conventional contra-directional coupler is operated the dimensions of the coupler, except for the length of the coupled sections of striplines 36, 38, generally do not vary with frequency.
Each of the dimensions of a coupler affects performance; for the purpose of providing tight coupling, the stripline spacing S.sub.1 is one of the important factors. For conventional TEM mode and quasi-TEM mode coupling, tight coupling requires (i) a spacing S.sub.1 on the order of thousandths of an inch with tolerances of ten-thousandths of an inch, and (ii) high impedance striplines in the coupled region (high impedance striplines are provided by selecting the dimensions of the striplines in the coupled region). In general, for TEM mode and quasi-TEM mode coupling the even mode impedance of the coupled sections of the striplines is approximately 120.OMEGA. for a 3B coupler, an increase of more than a factor of two (2) with respect to the 50.OMEGA. impedance of the remaining portions of the striplines and the first and second transmission lines.
Coupled striplines 12, 14 of some conventional directional couplers are constructed using metalized plastic layers similar to multi-layer printed circuit board. The metal is etched to form a desired conductor or circuit pattern. One type of coupler fabricated in this manner is a suspended substrate coupler in which the coupled striplines are suspended microstrips. A suspended coupler which operates as a quasi--TEM coupler is disclosed in Japanese Laid Open Application No. 62-114302--Miyazaki.
Very small, high-frequency geometries can be formed with suspended substrate couplers. However, until approximately 1984, it was believed that the maximum frequency which could be handled by coaxial couplers was 26GHz. This perceived limitation was related, at least in part, to the inability to manufacture components such as couplers with the small dimensions and tolerances required for frequencies above 26GHz. Tolerances on the order of approximately 0.0005" must be maintained for frequencies over 26GHz. Further, it has not been possible to make a coupler which provides coupling tighter than 3dB with conventional suspended substrate couplers.
At higher microwave frequencies (above 20GHz), suspended substrate couplers offer low loss, relatively large size features, and precise impedance control. For frequencies above 26GHz, the transmission lines must comprise microstrips provided on both sides of the suspended substrate because single sided transmission lines suffer from dielectric surface modes.
The directivity of contra-directional suspended substrate couplers is, in general, very poor for frequencies below 20GHz (directivity values usually range from 5 to 15dB). At frequencies above 20 GHz the directivity of a suspended substrate contra-directional coupler rapidly degrades further. In addition, conventional suspended substrate couplers suffer from losses due to, for example, line resistances and losses in the dielectric. At high frequencies these losses are more critical because losses generally occur per wavelength--the shorter wavelengths associated with higher frequencies result in more loss per length of transmission line. One cause of losses in conventional suspended substrate contra-directional couplers is the need to provide the transmission lines with an increased impedance--on the order of 120.OMEGA. for a 3dB coupler--in the coupling region in order to provide tight coupling.